Reciprocating bucky diaphragm



April 1952 c. K. GIERINGER ET AL 2,591,536

RECIPROCATING BUCKY DIAPHRAGM Filed Jan. 15, 1949 5 Sheets-Sheet l 97 EM ERlr! I l \\\\\\\\\\\v\\\\\\\\\\\\\\\- \\xfiw w? 5 INVENTORJ.

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ATfoRNEYi April 1952 c. K. GIERINGER ETAL 2,591,536

RECIPROCATING BUCKY DIAPHRAGM Filed Jan. 15, 1949 5 Sheets-Sheet 2 M I] I IIYVENTOR5- By v April 1, 1952 c. K. GiERlNGER ETAL RECIPROCATING BUCKY DIAPHRAGM 5 Sheets-Sheet 5 Filed Jan. 15. 1949 A RNEYS 5 ow pm. ow

- posure.

Patented Apr. 1, 1952 EECIPRQGATIN G BUCKY DIAPHRAGM:

Carl K. Gieringerand Richard Duckering, Cincmnati, Ohio, assignors to The Liebel-Flarsheim Company, Cincinnati, Ohio, a corpora- Application January 15, 1949, Serial. him-71,074

Claims.

This invention relates to apparatus for producing X-ray photographs. It is directed particularly to an improved mechanism for actuating a Bucky diaphragm of the type conventionally employed to screen secondary radiations from the photographic film or plate. The invention also is directed to a method of producing X-ray photographs which is adapted to be used for short or long exposures, as the case may be.

In substance, the invention is based upon reciprocation of a Bucky diaphragm at different rates of speed during an X-ray exposure. However, in order to fully understand the nature of the present invention and the benefits which are provided by this relatively simple concept, a brief review of roentgenology employing Bucky diaphragms is requisite.

. A conventional diaphragm, termed a Bucky diaphragm from the name of its inventor, consists of a plurality of strips of material which are opaque to X-rays and which are arranged, on edge, in spaced relationship to one another. The diaphragm is interposed between the subject to be X-rayed and the photographic film or plate. Primary radiations generated at the X-ray exposure, movement of the grid being started before the start of the. exposure and continuing momentarily after the exposure is terminated, to avoid any shadow effects.

Since various subjects require different exposure periods, because of their density and thickness or X-ray opacity, the usual Bucky mechanism is selectively controllable in rate of movement to meet the time conditions of exposure. Thus, the X-ray technician, by his judgment and experience, sets thev actuator to cause passage of the diaphragm in a period of time slightly longer than the exposure period he believes will be proper for the subect to be photographed; mechanisms presently available are adustable to.

' accommodate exposures of fromJ/ to 40 seconds,

tube pass through the subject and through the diaphragm to impinge upon the photographic film or plate, the slits of the diaphragm through which the X-rays may pass being angulated to coincide with the diverging stream of X-rays generated at the tube. However, in passing through a-subject, some X-rays are scattered or deflected, thereby producing so-called secondary radiations which would blur the image on the photographic plate if they were unobstructed. It is the function of the Bucky diaphragm to screen such secondary radiations from the photographic medium; this result is accomplished because the grid or diaphragm permits the direct radiations to pass through its slits but it excludes radiations which approach it along paths outside of the angle of acceptance delineated by its slits.

The opaque strips of a Bucky diaphragm would cast their shadows upon the film if the diaphragm were stationary during the X-ray ex- To avoid this result it is a conventional practice to move the diaphragm, at a uniform rate,'over the surface of the photographic medium during an exposure period. A Bucky actuator is employed for this purpose, which consists essentially of a motor mechanism, usually a spring, to force the diaphragm in one direction, and a governor which controls the speed of movement to the desired rate. The grid of such an apparatus, commonly termed a Potter-Bucky diaphragm, makes one pass over the film during the this range being adequate to accommodate the. X-ray examination of a common variety of sub,- jects. If the exposure is to be longer than a period of time within this range, then the total exposure period may be divided into two or more partial exposure periods, and after each partial exposure, the diaphragm actuator will be cooked and released for the second and succeeding partial exposures. However, such procedure obviously is inconvenient and uncertain.

A further difllculty is encountered in X-ray photography utilizing a diaphragm because of the pulsating nature of the X-ray emanations generated by the tube of the machine. Since such tubes are operated upon alternating current,

usually cycle, it is apparent that if an X-ray pulsation happens to occur, as it frequently does just when the X-ray opaque. strips of the diaphragm reach thev same relative position. that other opaque strips of the diaphragm occupied during preceeding pulsations, then the strips, though actually in movement, will cast shadows just as though they were stationary, and the resultant photograph will be imperfect. Such a condition is known in the art as synchronism, and will be referred to inthis specification and the claims hereof by that name. A non-synchronous condition is one in which the timing is such that coincidence of grid positions and X-ray pulsations will not occur to such extent that discernable shadows or grid lines will appear upon the resultant photograph.

Bucky actuators presently available usually are constructed to embody special provisions by which they may not inadvertently be set at. points where objectionable synchronous conditions will occur. However, the problem of avoiding synjchronism becomes much more acute when the apparatus is to be constructed to accommodate a wider, longer range of exposure periods.

The principal objective of this invention has been to provide a Bucky actuator adapted to serve both short exposures, e. g. 1 second, and exposures of a prolonged nature, for example to seconds or more, or, in fact, exposures which may be for as long a period of time as may be required by the subject undergoing photographic examination. A further principal objection of the invention has been to provide a mechanism which is substantially free of any latent tendencies to provide objectionable synchronous conditions through its movement either during long or short exposure periods.

The apparatus of the present invention is useful in conventional photographic technique wherein the operator, by his choice, or from chartsand data, arbitrarily selects the time for which a subject is to be exposed and sets his apparatus accordingly. However, the invention is particularly useful in X-ray photographic techniques more recently proposed, by which ex posure is governed automatically to the correct time through excitation of a photo cell; In roentgenography employing such photoelectric timing, a fluorescent screen is provided which emits light in proportion to the X-rays impinging upon it after they have passed through the photographic medium. A photo cell, focused upon this screen, generates a current in proportion to the light emitted from the screen, and such current is employed in the charging of a condenser in a control circuit. When the charge upon the condenser reaches a critical value, which is adjusted in respect to the known sensitivity of the photographic medium, the control circuit is energized to terminate the exposure. The arrangement therefore constitutes an automatic exposure meter which eliminates the need for the'operator to exercise his skill or judgment.

It will be apparent that a conventional singlepass Bucky diaphragm is very difiicult, if not impossible, to use successfully in photographing a variety of subjects with photo-timed X-ray equipment, since the operator has no means of determining the rate of movement of the diaphragm necessary to accommodate an unknown but automatically terminated exposure governed by the photo-timer. Reciprocating diaphragms adapted to be moved back and forth across the photo-medium at a uniform rate of speed have been proposed but the results have generally been unsatisfactory because of the incidence of synchronism and because imperfect pictures are produced when the exposure is terminated at a moment of reversal of the diaphragm.

This invention is based upon th concept and determination that synchronous conditions may be avoided, whether the exposures be for a short or long period of time, when the Bucky diaphragm is reciprocated during the exposure period, in back and forth directions at different rates of speed, such that the period of time re quired for completion of diaphragm movement in one direction is only a, fraction of the time required for its movement in the opposite direction. Otherwise expressed, the diaphragm, during exposure, is shuttled back and forth but travels in opposite directions at differential rates of speed, the fast rate of travel being suited for short exposure periods, the fast and slower rates of travel, in one whole or partial cycle of movement, being suited to accommodate longer exposures, and the travel of the diaphragm through a number of cycles of movement being suited to accommodate prolonged exposure however lon the actual time period might be.

The exact rates of speed at which the diaphragm is moved in opposite directions are of no critical importance, but preferably neither bears an integer or simple sub-multiple relationship to the pulsations of the X-ray tube. By way of illustration, but not by way of limitation, a diaphragm having a 2" movement may be advanced in one direction in 0.3 seconds, and returned in 2.1 seconds such that a complete cycle of movement will require approximately 2.4 seconds. These rates of travel have been found to give greatly improved results or with the use of Bucky diaphragms of both high and low rid ratios. The slowest rate of movement should be great enough to cause a sufiicient number of opaque strips of the diaphragm to pass a given point and thereby provide proper screening without shadow efiects. On the other hand, the higher rate of movement in the opposite direction should not be so great as to cause vibration of the equipment through momentum effects at the instance of reversal. Also, it is desirable that the rate of movement in either direction be substantially constant, once the diaphragm is placed in motion.

In a typical construction embodying the principles which have been just discussed, a spring and an electrically energized solenoid motor or thrustor are employed to actuate the Bucky in opposite directions. Inasmuch as both a spring and a solenoid or the like act promptly to cause reversal of motion when they are free to do so, the change of direction of diaphragm movement conveniently may be caused to' take place in approximately one one-hundredth of a second or less, which has been found to be sufficiently fast to prevent the formation of grid lines on the photographic medium during the momentary period of reversal while the grid is stationary.

The desired differential rate of movement of the grid of the diaphragm in opposite directions preferably is provided by a damping mechanism which limits the rate of movement in one direction but not the other. This damping mechanism by way of example may consist essentially of a fluid circuit including a cylinder containing a liquid, a piston operatively connected to the diaphragm, a bypass around the piston, and a fluid relief device capable of permitting the piston to move in the cylinder with greater ease in one directio than in the other.

The invention also contemplates an electric control system which is responsive to the movement of the diaphragm for periodically energizing the solenoid or thrustor as required to actuate the Bucky by this means. In addition, an actuator, grid holder, and cassette unit is provided employing a disk having a series of photoapertures any one of which conveniently may be selected for use at the time a given subject is to be photographed.

Further objects and other advantages of the present invention of which a typical embodiment is disclosed will be apparent. From the foregoing discussion of the principles of the invention and the following detailed desciption, those skilled in the art readily will comprehend various modifications to which the invention is susceptible.

In the drawings:

Figure l is a general top plan view of the apparatus.

Figure 2 is a sectional view taken on line 22, Figure 1, detailing a portion ofthe actuating mechanism for the Bucky diaphragm.

Figure 3 is a sectional view taken on line 33,

Figure- 1, illustrating the diaphragm actuating solenoid and damping apparatus.

Figure 4: is an enlarged longitudinal sectional view detailing the hydraulic damping unit.

Figure 5 is across sectional view taken online 5-5, Figure 4, detailing the regulating valve for the damping unit.

Figure 6 is a sectional view taken on line 6-E, Figure-4, detailing the damping piston.

Figure 7 is a sectional view taken on line ?-i, Figure 4, detailing the damping cylinder construction.

Figure 8 is an enlarged sectional view taken on line 8-8,.Figure 1, illustrating the Bucky diaphragm mounting and actuatingstructure, the film tray. and the aperture dish and its driving motor.

Figure 9 is a fraginentary sectional view taken on line 9-9, Figure 1, detailing the aperture disk indexing plunger, solenoid and the exposure switch and its association with the Bucky diaphragm'.

Figure 10 is a fragmentary sectional view taken on line lillll, Figure 1, illustrating the damping unit and the reversing switch for the diaphragm actuating solenoid and the actuating fingers for the switch.

Figure llis a fragmentary sectional view taken on line H--l I, Figure 8, looking upwardly from the bottom and illustrating the aperture. disk and its associatedmechanism.

Figure 12 is a diagrammatic view illustrating the operation of the hydraulic damping unit and exposure switch after reciprocation of the diaphragm is initiated.

Figure l3is a diagrammatic view illustrating the solenoid reversing switch in a position cor responding to that of Figure 12.

Figure 14 is a diagrammatic view similar Figure 13, showing the actuation of the reversing switch at the right hand: stroke limit of the diaphragm.

Figure 15 is a diagrammatic view similar to Figure 12, showing the operation of the damping unit on the returnstroke of the diaphragm.

Figure 16 is a graphillustrating the velocity curve of the diaphragm during one cycle of oper ation. V

Figure 17 is a diagrammatic view illustrating the electrical circuit for. the aperture selector and diaphragm reciprocating apparatus.

Described generally with reference to Figure 1, the Bucky diaphragm or grid. unit constitutes a rectangular sheet metal housing, indicated generally at 20, in which is slid-ably mounted the grid 2|, its actuating mechanism and the aperture selecting apparatus. The grid is reciprocated by a solenoid 22 and a tension spring 23, so arranged that the grid is drawn in one direction by the solenoid and returned by the spring. In order to provide sustained reciprocation dur ing an exposure period, a reversing switch 25 is electrically interconnected with the solenoid and the switch is intermittently opened and closed bya switch tripping element 25 mounted upon the grid. When the solenoid is deenergized, the grid bottoms against one side of the housing with the reversing switch closed, actuation being initiated by a push button switch 2 5 (Figure 1'7), which is depressed by the operator during the exposure period. This switch energizes the solenoid circuit causing the grid to move to the right, placing the solenoid under the control of the reversing switch so long as the button is held in depressed position. The reversing switch is of the snap type havinga pair of spaced trip elements which, determine the stroke. of the grid, these being alternately contacted by the trippingelement 25 such that the circuit is closed when the grid reaches a left hand stroke limit but before it bottoms against the housing, and opened when the grid reaches its right hand stroke limit. Thus, the reciprocation is continued so long as the push button switch is held in depressed posi tion. When the solenoid circuit is opened at the end of the exposure period as hereinafter disclosed, the grid returns to its left hand position of rest against housing 20.

In order to control the velocity of the grid during its return stroke under spring tension, there is provided a-hydraulic-damping unit indicated generally at 21, including apiston having a rod connected to the. grid andoperable to oppose the tension exerted by spring 23, thereby to control the speed of thegrid during its travel to the left. The grid therefore is reciprocated in one direction by the solenoid and in the opposite direction by the tension spring and the combined effect of these two elements is modified by the damping unit 2? to produce a velocity curve similar to that indicated in the diagram illustrated in Figure 16.

In the present apparatus theexposure period is controlled by a photoelectric timer which controls automatically the time period for each individual exposure. The timer forms no part of the present invention and is not disclosed, but its successful operation depends upon the aperture selecting apparatus embodied in the grid assembly. In general, the timer constitutes a detector unit and control-system, the detector unit including a fluorescent screen so arranged that when roentgen rays 'fall. upon the screen, it emits visible light rays which strike a phototube to cause passage of electric current. The current passed by the tubeisfed toa condenser in the control circuit and when fully charged, the condenser discharges andactivates the control circuit to shut down the X-ray tube. The time required to charge the condenser determines the length of exposure and the apparatus is arranged so that a uniform exposure period is applied independently of the density of the medium through which the roentgen rays pass. In other words, the photographic exposure is equally dense whether the rays pass through a relatively transparent portion of the body or through a more opaque area since the condenser charge is proportionate to the amount of radiation passing through the subject and film.

In operation, an aperture disk, indicated'generaliy at 28 (Figure 1), made of X-ray opaque material, for example, laminated lead and aluminum sheets, is rotated to align one of several apertures or stops with the fluorescent screen, the size of the aperture being selected according to the part of the body to be photographed. Drdinarily, if a small part, such as the wrist-or knee, were X-rayed without masking the sur rounding area, a considerable amount of v radiation would pass around the region of interest, shortening the exposure period and rendering the timer unreliable. The aperture disk is opaque to roentgen rays and the apertures are of graduated size so that a small aperture may be utilized for photographing small areas and larger aper tures for larger areas, thereby rendering the timing function substantially constant.- By this arrangement, the timing apparatus is energized only by the rays passing through thepart being photographed and the density of that part determines the exposure period, providing uniform results automatically without adjustment by the operator.

In operation, the operator depresses the start button 28 which energizes the-solenoid to start reciprocation of the grid. As soon as the grid 7 begins to move to the right as shown in Figure 1, it closes the exposure switch 39 which is in electrical connection with the X-ray apparatus to excite the X-ray tube. The operator holds the push button in depressed position and the diaphragm continues to reciprocate and the X- ray tube remains active until the timer charges its condenser to the critical point, whereupon the condenser discharges and shuts down the X-ray tube, terminating the exposure.

Bucky actuating mechanism As shown in Figure l, the housing 2& constitutes a base plate 3| having a marginal flange 32 which includes an inturned lip 33 at its upper edge. The Bucky grid 2i constitutes the usual arrangement of vertically disposed lead strips providing slits through which the X-rays pass to the photographic plate. This structure is not disclosed in the drawings since it follows conventional practice. The diaphragm is enclosed by a marginal frame 34 formed of channel shaped sheet metal strips suitably joined together at the corners. The forward edge of the grid assembly is slidably mounted upon a pair of rods 35-35, each supported by a pair of brackets 35-35 which are secured upon a framing member 37 extending across the frame 23 (Figures 1 and 2). The brackets 36 are U-shaped in cross section and the rods 35 are secured by means of set screws 38 passing through the brackets and engaged against the rods 35. The lower ends of the brackets 36 are secured to the framing member 31 preferably by spot welding. The Bucky grid assembly is slidably mounted relative to the rods 35 by means of bearing blocks 40- 18 secured by means of screws 4l'to the frame 34, the bearing blocks being drilled to provide a sliding fit with respect to the rods 35. The rearward edge of the diaphragm 2| is slidably supported by bearing blocks 42 similar to the blocks 49 but provided with aslot slidably engaged upon a slide rail 43, secured by means of angle brackets 4444 to the rear flange 32, as shown in Figures 1 and 8. The bearing blocks 40 and 42 thus slidably maintain the Bucky grid in a vertical plane and permit it to be reciprocated laterally with respect to the housing 20.

The grid actuating solenoid 212 is a commercial product and therefore need not be disclosed in detail. In general, it constitutes a coil 4 (Figure 17) enclosed by a housing 46 (Figures 1 and 3), the housing being secured upon the base plate 3!. The solenoid includes the usual plunger 41 slidably extending through the coil 45, the plunger or core 41 being connected to the diaphragm by means of a bracket 48 having angular limbs 5050 at opposite ends, secured to the opposite ends of the plunger 41. The bracket 48 is secured to the frame of the diaphragm by means of screws 5| passing through the bracket into the right hand bearing block 40 and into an intermediate block 52 which is secured by screws to the grid frame.

It will be apparent that when the solenoid is energized, the plunger 41 will be retracted toward the right, as viewed in Figure 1, to move the grid to the right against the tension of spring 23. The

8 solenoid is energized and deenergized automatically by the switch 24 which is actuated by the trip element 25 attached to the grid frame. The operation of this switch will be more clearly disclosed with reference to the electrical circuit, it being sufiicient to note at this point that the switch is closed when the grid is in its left hand position and is opened when the Bucky attains its right hand limit of travel, at which time the solenoid is deenergized and the spring 23 returns it to, its left hand position. As shown in Figure 2, the left hand end of spring 23 is anchored upon thelower end of the left hand rod support bracket 38 and the opposite end of the spring is anchored upon a pin 55 secured to the right hand bearing block 40.

In the absence of the damping unit 21, the grid would reciprocate at a high velocity such that undesirable vibrations would be developed, resulting in a tendency for grid lines to be visible in the photograph. As shown in Figures 1, 3 and 4, the damping unit 21 constitutes a cylinder 54 having a piston 55, the piston including a rod 53 connected by means of nuts 57 to a plate 58. Plate 53 is connected by means of rods 59 to a plate 68 secured upon the end of the solenoid plunger 41 by means of a nut 6!. It will be apparent therefore that movements of the solenoid plunger ii are transmitted to the piston 55 so that the rate of reciprocation of the grid is regulated by the damping unit. As shown in Figures 3 and 15, a clearance bore 51a is provided in plate 58 to permit the piston rod to float laterally relative to the piate, there being a slight amount of clearance between the, nuts 51 and plate 58. The rod thus floats laterally with respect to the plate to permit it to align itself in its cylinder. If desired, the velocity. curve may be modified at the reversal points by providing a substantial amount of clearance between the nuts and plate.

Described in detail, the damping unit cylinder 54 includes cylinder heads 53 and (it at, opposite ends, these including bypass openings 55765 communicating with a bypass tube 66, with'a control valve 6? located within the head 63, to regulate the operation of the piston.v As shown in Figure 4, the right hand cylinder head 64 includes a packing gland E8 to provide a sliding sealed fit with the piston rod 58. The unit is filled completely with fluid such as oil and in order to compensate for the oil displaced by the piston rod as it enters the cylinder, an expensible metal bellows Ill (Figure 3) is provided. Bellows 70 is formed from flexible sheet metal and is free to expand longitudinally as the piston rod passes into the cylinder. The bellows also compensates for expansion of the oil due to temperature changes. As shown, the bellows includes a stem H which is slidably supported by a bracket 12 (Figures 3 and 5) topermit expansion of the bellows.

When the solenoid is energized, the piston rod 56 will be drawn to the right as viewed in Figure 4. In order to attain relatively high velocity during this stroke, the fluid bypasses through the piston 55 as well as through the bypass line 56. For this purpose the piston rod includes a valve element 13 which seats upon an intermediate wall 14 formed with the piston 55. The valve seat 13 is screwthreaded upon the end of the piston rod and normally is held in closed position by means of a compression spring '15 seated against the intermediate wall 14 and having its opposite end seated against a head 16. The head 76 includes a square, nut portion 11,

9 the corners of which overlie the-end of the piston as shown in Figure 6, providing openings '18 for the passage of fluid through the piston. As shown, a spacer 80 is disposed between the valve seat and head so that a predetermined amount of clearance may be maintained between the valve and its seat.

It will be apparent that when th solenoid is energized drawing the piston rod to the right, the spring 75 will be compressed until the head 71 seats against the piston with the valve 13 unseated. Thus, fluid will bypass through the piston during the right hand movement of the grid. In order to attain the desired velocity, it has been found that a valve unseating movement of approximately .015 is desirable, and this-clearance may be obtained conveniently by spacing the squared head 11 from the end of the piston, as indicated at 8|, when the valve 13 is in closed position. During the right hand movement of the grid it will be observed that a portion of the fluid also will bypass through the bypassing tube 66.

At the right hand end of cylinder 54 there is provided a series of slots 82 milled into th bore of the cylinder. The purpose-of these slots is to provideuniform velocity at the beginning of the-stroke under spring tension, after the solenoid is deenergized. The slots 82 provide bypass openings around the piston when it is v in its right hand position, to speed up the initial return movement. As the piston passes the slots, the grid has attainedfull speed, the valve 13 being closed as shown in Figure 4 with the'oil bypassing .through the control valve 61.

As detailed in Figure 5, the regulating valve 61 constitutes a core 83 which is rotatable within a bore formed in the left hand cylinder head 53. The valve includes an adjusting stem 84 which is sealed with respect to the cylinder head i core 83 further is provided with a circumferential groove 88 (Figure 5), of gradually increasing depth, so that a fine adjustment may be had by rotating the stem 84.. The structure is arranged to bepermanently adjusted by the, manufacturer to provide the proper grid velocity and for this reason the valve stem 84 is not provided with an adjusting knob, thou h itmay beif desired. As shown in Figure 3, the damping unit is secured I inposition by means of screws 90 passing through the base plate 3| into lugs 9I-9l forming a p rt of the assembly.

Operation of grid actuating apparatus The-operation of the apparatus is diagrammatically illustrated in Figuresl2 to 16 inclusive.

. As shown in Figure 12,, thepush button for energizing the apparatus has been depressed'and the grid is beginning its right hand reciprocation as indicated by the arrows. switch 24 is illustrated diagrammatically in Fig- The solenoid control ure 13 in a position corresponding to the position of the grid in Figure 12 with the trip element also travelling to the right. This switch is a commercial product such as a micro-switch of the self-holding type so. arranged that-its switch blade 92 snaps either to a closed or open position by depressingeither of the actuating plungers 93 or -94. When the grid is at rest, it is positioned against the. stop 95 as shown in Figure 1, with the areas 10 lower finger 25a in switch closing position against the lower switch trigger .95, as shown in broken lines (Figure 13). When the start button is depressed, switch plunger '93 remains in circuit closing position until the upper switch trigger!" is engaged'by the upper finger 25b, as shown in Figure 14, at which time the switch is snapped to an open position to deenergize the solenoid and permit the grid to return under the tension of spring23.

The triggers 9'6 and "97. are in the form of flat springs, anchored as .at '98 on the switch and each includes an angular portion I00 to provide a camming action with respect. to the trigger fingers 25a and 252). When the grid reaches the left hand limit of its travel, the lower finger'25a will again contact the lower switch trigger 96 to close the switch and energize the solenoid. It is to be noted that the grid is reversed before it returns to its position of res1t against the stops 95, since the range of reciprocationis governed by the switch triggers 96 and 91, in combination with the fingers 25a and 25b. When the circuit is deenergized, the grid again returns to it 'left hand position and the lower finger 25a assumes the position shown by the broken lines in Figure 13.

The operation of the damping unit 21, during the right hand stroke of the grid, is diagrammatically illustrated inFigure 12 As shown, the valve element 13 is unseated and the fluid is bypassed through the piston, as indicated by th arrows. When the solenoid is deenergized, the valve 13 will seat as indicated diagrammatically in Figure 15, so that the fluid must bypass through .the bypass line 65 and metering valve 61, as indicated by the arrows. This regulates the return velocity of the grid as previouslynoted. The apparatus as disclosed is arranged to provide an operating stroke of approximately two inches, at diiferential speeds for the right and left reciprocations, as illustrated diagrammatically in Figure 16. The operation of the'reversing switch 24 causes reversal of the grid with sufficient abruptness to avoid formation of grid lines'and the forward and return'velocity is non-synchronous so that succeeding grids do not occupy the same relative position at succeeding impulses of the X-ray tube. Theselected velocities illustrated are based upon a given frequency and grid line spacing, but the -metering valve 61 may be a'djusted'to adapt the apparatus to finer or coarser grid spacings and other current frequencies if necessary. As'indicated at l0! on the diagram, the reversal points occupy lessthan of a second,

which issufiicient to avoid photographin grid lines. An important advantage of the velocity curve disclosed is that "the solenoid is energized for a rather brief interval as compared with the interval of deenergization, thereby preventing it from becoming-overheated.

It will be noted inFigures l2 and 15 that the X-ray control switchtllis closed as soon as the grid makes its first pass to th right and remains closedso long as'the grid is eciprocated under control of the 'reversingswitch'z. After having been energizedby the control switch, the X-ray circuit remains energized until deenergize'd automatically by the phototube control system, as above noted.

Aperture selector mechanism As shown in'Figure 8, the housing 20 includes a-tray I02 disposed immediately beneath ,the grid 2| adapted to receive the film cassette (not ,the disk. to rotate.

ear-at shown), the casing being open as at I03 on one or both sides of the tray to receive the cassette. Tray I02 is made from sheet metal for example, steel sheets, and is provided with a central opening I04 through which the rays may pass to impinge upon the fluorescent screen of the timer unitv (not shown). As disclosed in Figure 8, the aperture disk 28 is disposed beneath the aperture I04 and'the disk is provided with a small aperture I05, medium sized aperture I and a large aperture I07. These apertures or stops are located 120 apart and are selectively aligned with the opening I04 by rotating the disk 28. The apertures are selected electrically by means of a hand operated selector switch I08 (Figure 17) which energizes an electric motor I09 to rotate and index the disk and align the selected aperture with the opening I04.

Described in detail with reference to Figures 8 to 11 inclusive, motor I00 is mounted upon an angle bracket IIO secured to the housing 20. The shaft of the motor I09 includes a worm iII meshing with a worm wheel II2, keyed upon a 'shaft H3 (Figures 1 and 8), which is loosely journalled upon bracket IIO. Upon the lower end of shaft II3 there is keyed a grooved pulley .II4 engaging a belt H5 which passes around the aperture disk 28. The worm and worm wheel provides a speed reduction unit between the motor and disk'such that v.the disk is rotated at a relatively slow rate of speed. As shown in Figure 11, belt H5 is kept taut by means of an idler pulley IIE journalled upon an arm II'I pivotally mounted as at II8 upon an intermediate plate I which is mounted beneath the film tray I02.

,A- tension spring I2I is anchored upon an angular finger I22, forming a part of lever II! and the opposite end of the spring is anchored upon a pin I23 secured to the housing 20.

The disk 28 is opaque to X-ray radiations for which purpose it is fabricated from alternate layers of aluminum and lead sheets. The disk is loosely journalled upon a screw I24 engaged in a plate I25 which extends across the opening I255 formed in the plate I20 and the disk is rein- ,forced by means of a hub I21 surrounding the screw I24. As shownin Figures 9 and 11, the disk is indexed by means of a plunger I28 in conjunction with pins I30 so arranged that the respective disk apertures are selectively aligned with the aperture I04. Plunger I28 is slidably mounted by a U-shaped bracket I3I adjacent the disk 28, secured to the plate I20. The outer end of the plunger is slidably supported by a stud I32 passing through a slot 133 formed in the plunger. The plunger is actuated by means of a solenoid I34 secured to the housing 20 as shown in Figure 9. When the solenoid is energized,

the end oi the plunger I210 is retracted out of the path of movement of the pins I30, permitting When the solenoid is deenergized, the plunger is advanced to its pin engaging position by a tension spring I35 having one end anchored upon a pin I30 secured upon ,theplunger and havin its opposite end anchored upon a bracket I31 which is mounted upon the switch 30 as shown in Figures 1 and 9. As shown in Figures 2 and 9, the switch 30 is supported upon a U-shaped bracket I38 to support the switch and bracket at the proper elevation.

When the selector switch I08 is actuated to rotate the aperture disk, a circuit is established to theidriving motor I00 and to the solenoid I34 so .that'the plunger'is retracted to permit rotation of the disk. The disk 28 is indexed by means of the three indexing switches I40 which are actuated by means of a cam block I4I secured upon the circumference of the aperture disk. The electrical circuit by means of which the in dexing operation is performed is hereinafter disclosed with reference to the circuit diagram illustrated in Figure 17. The switches I40 are mounted upon the plate I20 at apart, the switches being of the single pole double throw type, and may be of any commercially available form such as micro-switches. As shown in Figure 11, each switch includes an actuating arm I42 pivoted as at I43, the opposite end of the arm having a roller I44 adapted to be engaged by the cam block or skid I4I. As shown, the leading edge of cam block I4I is inclined or angulated as at I45 for camming engagement against the roller. When the switch armior the selected aperture is depressed by the block I4I as shown in Figure 11, the circuit to the motor and solenoid is broken to cause the motor to stop and the solenoid plunger to advance to'its indexing position. The arrangement is such that the motor I00 is shut down and the solenoid deenergized slightly before the pin I30 is in its final position, whereby the momentum of the motor carries the pin into engagement with the plunger. By virtue of the switches and plunger, the alignment of the apertures is positive, therefore slippage of belt H5 has no effect on indexing. As shown in Figure 8, the base p1ate 3I of- Electrical control circuit Figure 17 illustrates the circuit for the aperture selector apparatus in conjunction with the grid actuating circuit. As shown, the apparatus is energized by the A. C. input lines, indicated at I41 and I40. Line I4! is connected to the selector switch I08 and the other line I48 completes the circuit to the indexing solenoid I34, indexing motor I09, grid actuating solenoid 45, and the X-ray control circuit. Since the X-ray control circuit forms no part of the present invention, it has not been illustrated in detail. The X-ray circuit, as previously noted, includes the photoelectric timer which deenergizes the X-ray apparatus after the timer has been energized for a predetermined period. As illustrated, the selector switch I00 is set to index the small aperture I05 to its operating position.

It will be noted that the three positions for the selector switch are indicated at S, M, and L, and the corresponding indexing switches I40 also are indicated at S, M, and L, respectively for the small, medium and large apertures. The selector apparatus includes a corresponding pilot lamp I49 for the respective apertures which indicate to the operator that the apparatus is in condition for before the motor stops. bypasses from M contact through the pilot lamp type bridge rectifier inoperative position and the circuit is completed through selector switch I08, line I50, through the pilot lamp I40, through line II, which completes the circuit through motor H and solenoid I34 to thepower line I48, to illuminate the small aperture pilot lamp. As hereinafter described. in detail, each switch I includes two contacts, one for energizing the motor I09 and solenoid I34 and the other for completing the circuit to the grid actuating switch 25 after indexing.

lit the selector switch is rotated to the M contact, the circuit will be completed from line I4! through the switch to the M contact to energize line I52, which extends to the pole of M switch I40. As shown, the pole of M switch completes the circuit to the normally closed contact which is in connection with the line I53, extending to line IEI through motor I09 to the opposite power line I48. There also is provided a branch line I54 extending from line I5I to the winding of the indexing solenoid I34 so that the motor and solenoid are energized simultaneously when the selector switch I08 is moved to the M contact. When the selector switch moves oiT of the S contact, the pilot lamp for the small aperture is deenergized.

As motor I09 operates, the disk will rotate in the direction indicated by the arrow until the cam plate I4I reaches the M switch I40, moving the pole to its normally open contact which is connected to branch line I55. When the M switch is tripped by cam plate I4I, it, of course, deenergizes indexingsolenoid I34 and motor I 09, causing the solenoid plunger I28 to advance into indexing position with respect to the pin I30, just At this time the circuit I49 for the medium opening, indicating to the operator that the apparatus is in condition for making an exposure. The circuit for the grid solenoid is, of course, completed through the selector switch I08 to the line I52, through M switch to line I to one side of the push button 26 so that the grid can be actuated after indexing. From the push button the circuit is completed through line I55, reversing switch 24 to the solenoid 45. It is to be noted at this point that the solenoid 45 is equipped with a rectifier circuit which, as indicated, constitutes a plate I57 and condenser I58. These elements are mounted within the housing 20 as shown in Figure 1.

When push button 26 is depressed to reciprocate the grid, branch line I59 to the exposure starting switch 30 is energized. As shown, the switch normally is in open position, the switch arm I60 of'switch 30 having a roller IfiI at its free-end which is actuated by a cam block I52 secured to the edge of the grid frame. Switch 30 closes as soon as block I62 disengages the roller I 6i thereby completing the circuit to the X-ray control circuit. The reciprocation of the grid is within the range provided by the switch 24 during which period the exposure starting switch 30 remains closed. When a sufficient period of exposure has elapsed, the exposure meter will automatically deenergize the X-ray circuit and this equipment is provided with indicating means to inform the operator that the exposure is completed at which time he releases the button 26 and removes the exposed film for development.

The circuit for the large aperture is' the same as that described with reference to the small and medium apertures, the L contact having a line I63 extending to the L aperture switch I40 and branching from the switch to the line I54 to the line I54 to energize the solenoid I34 and motor I09. After the disk is indexed, the L switch pole will be moved to its second position to energize line I55 to complete the circuit to the switch 26. After indexing, the pilot lamp for the L aperture is illuminated from the L contact through the lamp to line I5I through the windings of the motor and solenoid, as previously disclosed.

It will be observed from the foregoing that the aperture selector apparatus is electrically interlocked with the grid reciprocating apparatus by means of switches I40 so that it is impossible to energize the grid reciprocator solenoid until the aperture disk has been indexed. Since exposure switch 36 is closed only after the grids reciprocation has been initiated, it is impossible to energize the X-ray control circuit while the grid is stationary. This avoids the possibility of'exposing the photographic film to grid lines. By virtue of the selector apparatus, the operator'is able to select the desired aperture conveniently and quickly, depending upon the part of the body to be photographed, and after the selected aperture is positioned, the appropriate pilot lamp is energized to indicate that the apparatus is in condition to make the exposure. The push button 26 is then held depressed for an exposure period and the photoelectric timer automatically controls the period of exposure thus rendering the operation convenient and simple. If the grid reciprocating apparatus is to be operated without the aperture selector apparatus, the switch 26 is'connected to the power line I41 by means of the broken line I65 bypassing the indexing circuit. Whenthe circuit is so arranged, the operation of the X-ray control circuit and exposure switch 30 is the same as above disclosed except that aperture selection is omitted.

Having described our invention, we claim:

1. In the art ofmaking X-ray photographs, a method of screening secondary radiations, emitted from a subject, from a photosensitive medium, the method which comprises energizing an X-ray tube to eifect pulsating discharges therefrom at a given rate, and reciprocating a Bucky diaphragm over the face of a photosensitive me"- dium in opposite directions at respectively constant velocities which are different from one another and which are non-synchronous with respect to the rate of pulsation of the discharges from the X-ray tube both of said constant velocities prevailing, respectively, throughout substantially the entire range of movement of the Bucky diaphragm in said opposite directions.

2. In the art of making X-ray photographs, a method of screening secondary radiations emitted from a subject from a photosensitive medium, the method which comprises energizing an X-ray tube to effect pulsating discharges therefrom at a given rate, and reciprocating a Bucky diaphragm over the face of a photosensitive medium in opposite directions at different rates of movement which are non-synchronous with respect to the rates of pulsation of the X-ray tube the rate of movement of the Bucky diaphragm in one direction being substantially greaterthan the rate of movement in the opposite direction the respective rates of movement in said opposite directions being substantially uniform throughout the periods of movement of the Bucky diaphragm in said opposite directions.

3. A method of making X-ray photographs which comprises energizing an'X-ray tube to, effect pulsating discharges therefrom at a given rate, exposing an X-ray sensitive photo-medium to such X-ray discharges passing through a sub ject, and screening secondaiy radiations from said medium during the exposure by means of a Bucky diaphragm which is constantly reciprocated in opposite directions at different rates of speed which are respectively non-synchronous with respect to the X-ray pulsations and maintaining uniformity in the rate of movement of the Bucky diaphragm in each, direction.

4. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a slidably mounted Bucky grid, electrical power means operable to shift the grid in one direction at a uniform rate of speed, mechanical power means biasing said electrical power means operable to shift the grid in the opposite direction at a uniform rate of speed, and means operable inter mittently to energize and deenergize the elec trical' power means to cause sustained recipro" cation of the grid the said electrical and mechanical power means being coordinated in respect to one another to effect movement of said grid in said opposite directions at different rates of speed.

5. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a slidably mounted Bucky grid, electrical power means operable to shift the grid in one direction at a given rate of speed, mechanical power means including a damping unit biasing said electrical power means operable to shift the grid in the opposite direction at a different rate of speed, and means operable intermittently to energize and deenergize the electrical power means to cause sustained reciprocation of the grid.

6. An apparatus for reciprocating a Bucky grid continuously during an X-ray exposure comprising a slidably mounted Bucky grid, electrical power means operable to shift the grid in one 1 direction, mechanical power means biasing said electrical power means operable to shift the grid in the opposite direction when the electrical power means is deenergized, a switch operated by the grid intermittently to energize and deenergize the electrical power means to cause sus tained reciprocation of the grid, and a damping unit connected to the grid operable to provide a different rate of movement for the opposite strokes of the grid.

'7. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a housing having a Bucky grid slidably mounted therein, an electrical component operable to shift the grid during one stroke of its cycle, spring means biasing said electrical component operable to complete the cycle when the electrical component is deenergized, a switch mounted within the housing in electrical connection with the electrical component, a switch actuating member mounted upon the Bucky grid operable to energize and I deenergize the electrical component and a dampid ential rates of movement for the forward and return strokes of the grid.

9. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a slidably mounted Bucky grid, electrical power means operable to shift the grid in one direction at a given rate of speed, mechanical power means including a damping unit biasing said electrical power means operable to shift the grid in the opposite direction at a different rate of speed, and means operable intermittently to energize and deenergize the electrical power means to cause sustained reciprocation of the grid, said damping unit constituting a cylinder and piston, the piston being arranged to bypass fluid in one direction of grid movement to provide a different rate of movement in opposite directions. I I

10. A Bucky grid unit for an X-ray machine comprising a housing having a Bucky grid slidably mounted therein, a solenoid mounted within the housing having a plunger connected to the grid to actuate the same, spring means biasing said solenoid, a reversing switch in electrical connection with the solenoid, a switch actuating member mounted upon the grid to open and close said switch intermittently to energize and deenergize the solenoid to cause sustained reciprocation of the grid during an exposure period, and a damping unit connected to the grid operable to provide differential rates of movement for-the forward and return strokes of the grid. i

11. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a grid having an electrical power unit for reciprocating the same, a manually operated grid control switch for energizing the power unit, a rotatable aperture selector disk associated with the grid having a motor for indexing the same, indexing switches associated with the aperture disk for deenergizing the motor when the disk is rotated to a selected position, said switches being interconnected with said grid control switch to-deenergize the electrical power unit until the aperture disk is in a selected position. i

12. A Bucky grid unit comprising a grid having an electrical power unit for reciprocating the same, a manually operated grid control switch electrically connected to the grid power unit, a rotatable aperture selector disk associated with the grid having a motor for indexing the same, a manually operated selector switch interconnected with said motor for energizing the motor, indexing switches associated with the aperture disk and in electrical connection with the motor for deenergizing the motor when the disk is rotated to a selected position, said switches being interconnected with said grid control switch to deenergize the electrical power unit until the aperture disk is in a selected position.

13. A Bucky grid unit comprising a grid having an electrical power unit for reciprocating the same, a reversing switch mechanically associated with the grid and in electrical connection with the power unit to intermittently energize and deenergize the power unit and reciprocate the grid, a manually operated grid control switch in electrical connection with said reversing switch to control reciprocation of grid, a rotatable aperture selector disk associated with the grid having a motor for rotating the same to selected positions, a manually operated selector switch in electrical connection with said motor for energizing the motor, indexing switches associated with the aperture disk and in electrical connection with the motor for deenergizingthe motor when the diskis rotated to aselected position, said switches being interconnected with said manually actuatedgrid control switch to deenergize the electrical power unit until the aperture disk is in a selected position.

14. A Bucky grid unit comprising a housing having a grid slidably mounted therein, power means for reciprocating the grid relative to the housing, an X-ray opaque film tray mounted within the housing beneath the grid having an opening for passage of X-ray radiations, an X- ray opaque base plate beneath the film tray having an opening in registry with the opening in the film tray, an X-ray opaque aperture selector disk rotatably mounted between the film tray and base plate, the disk having graduated openings adapted to register with said openings, and means for rotating the aperture selector disk relative to said openings to index said apertures selectively with respect to said openings.

'15. A Bucky grid unit comprising a housing having a grid slidably mounted therein, power means for reciprocating the gridrelativa to the housing, a film support mounted within the housing beneath the grid, a base plate beneath I the film support having an opening therein, an aperture selector disk havingaplurality of apertures rotatably'mounted between the film support and base "plate, a'motor in driving connectionwith the selector disk for rotating the disk relative to said opening to index said apertures with respect to the opening and a selector switch for energizing said motor.

16. A Bucky grid unit comprising a housing having-a grid slidably mounted therein, power Q means for reciprocating the grid relative to the housing, a film support mounted within the housing beneath the grid, a base plate beneath the film tray having an opening therein, an aperture-selector disk having a plurality of apertures rotatably mounted between the film support and base plate, a motor for rotating the aperture selector disk relative tosaid opening to index said apertures with respect to said opening, a belt in driving connection with .said motor passing around the'aperture disk, an electrically operated stop plunger ,engageable with stop elements on the selector disk 'to'index the same, a selector switch interconnected with said motor and electrically operatedplunger to energize the motor and disengage the plunger simultaneously, and indexing switches mechanically associated with said disk and electrically interconnecting the motor and plunger to deenergize the same when a selected aperture is indexed.

17. A Bucky grid unit comprising a housing having a Bucky grid slidablymounted therein, said housing having a base plate including an opening adapted to receive a photoelectric timing unit, an aperture disk mounted above said base plate having a series of graduated apertures adaptedto be placed in registry with said opening by rotating said disk, an electrical motor in driving connection with said disk for rotating the same, a. selector switch for energizing said motor, apluralityof indexing switches associated with said dis'k,,n1eans on said diskior actuating said indexing switches when the selected aperture is in registry with saidppening, a solenoid operated plungerassociated with said disk, the disk having a plurality of pins adapted to be engaged by said plunger, said motor and solenoid being electri cally connected to said switches to index the disk when the motor and solenoid are deenergized by one of said switches.

18. A Bucky grid unit comprising a housing having Bucky grid slidably-mountedztherein, an electrical power llIlittfOI' actuating :the grid :in one direction at a given rate of speed, spring means for moving-the gr-id in an opposite'direction at a different rate of speed, a reversing switch mechanically interconnected-withthe grid for intermittently energizing the electricalpower unit, an .aperture selector disk mounted within said housing, the disk having -a plurality of graduated apertures, a motor for rotating said disk to index .the apertures selectively to anoperative position, a series of index switches associated with the disk and interconnected with said motor to deenergize the same when a selected aperture is indexed, .a manual switch for energizing the grid reversing switch to initiate -reciprocation of the grid, said indexingiswitches each having a contactin electrical connection with said manualswitch .operableto complete an electrical circuit to the manualqsw-itchto energize the .grid power unitafter the-index switch is actuated to deenergize the disk 'indexingmotor.

19. A Bucky grid unit comprising a housing having a Bucky grid slidably mounted therein, an electrical power unit for vactuating the grid in one directionat agiven rate of speed, spring means vfor moving the grid in an opposite direction at a different rate -.of speed, a reversing switch mechanically interconnected with the grid for .intermittently energizing the electrical power unit, anlaperture s'elec'tordiskmounted within said housing, the disk having aplur-ality of graduated apertures, amotor for rotatingsaid disk to index .the apertures selectively to an operative position, a series .ofindexswitches associated with the disk and interconnected with said motor'to deenergizelthe same when a selected aperture is indexed, ,a manual switch -for energizing the 'grid ,reversingswitch-to initiate reciprocation of the grid said indexing-switches each having .a contact in electrical connect-ion with said manual switch operable tocomplete an electrical circuit to ;the"manual switch to energize the grid power unit afterthe index-switch is actuated to deenergize the disk indexing motor, and a normally open .X-ray control switch mechanically interconnected with the grid, said switchadaptedto becloseddu'ring reciprocation of the grid to energize an X-raycircuit and adaptedto "be opened when the grid reciprocation stops.

20. An apparatusfor continuously reciprocating a Bucky grid during an X-ray exposure comprising a base having a Bucky gg-r id slidably mounted therein, a solenoid mountedon'the base having a plunger connected "to the grid, a tension spring having an and anchored on said base and having its opposite end connected to the grid whereby the grid is reciprocated in one stroke of its-cycle by thesolenoid' and inits -return stroke by the spring, a reversing switch interconnected with the solenoid iorintermittently energizing the 'same,'a..'switch actuating finger mounted on the grid for-actuating said switch, a damping unitmountedlon sa'idlbase and connected to the grid, *thedamping unit being adapted to provide a rate oftravel during solenoid operation substantially greaterthan the rate during spring operation whereby'the solenoid is deenerg-ized for a :cooling period after each power stroke of'greater-'durationxthan the power stroke.

21. A mechanism for reciprocating a Bucky grid continuously during an X-ray exposure comprising a base having means for slidably mounting a Bucky grid therein, an electrical power unit mounted in the base and connected to the grid, spring means biasing said power unit, the power unit being arranged to reciprocate the grind during one stroke of its cycle with the spring means arranged to complete the cycle when the electrical component is deenergized, a reversing switch in electrical connection with the power unit, a switch actuating member secured to the grid, the switch having a pair of spaced actuating elements cooperating with the actuating member to define the range of grid reciprocation, a manual control switch in electrical connection with said reversing switch, the grid being arranged to bottom against said base when the manual control switch is open, the reversing switch being arranged to reciprocate the grid within the range determined by the switch without bottoming the grid against the base during reciprocation.

22, A mechanism for reciprocating a Bucky grid continuously during an X-ray exposure comprising a base having means for slidably mounting a Bucky grid therein, an electrical power unit mounted in the base and connected to the grid,

spring means biasing said power unit, the power unit being arranged to reciprocate the grid during one stroke of its cycle with the spring means arranged to complete the cycle when the electrical component is deenergized, a reversing switch in electrical connection with the power unit, a switch actuating member secured to the grid, the switch having a pair of spaced actuating elements cooperating with the actuating member to define the range of grid reciprocation, a manual control switch in electrical connection with said reversing switch, the grid being arranged to bottom against said base when the manual control switch is open, the reversing switch being arranged to reciprocate the grid within the range determined by the switch without bottoming the grid against the base during reciprocation, and an X-ray control switch mechanically interconnected with the grid and electrically interconnected with an X-ray circuit to deenergize automatically the X-ray circuit when the grid is bottomed against the base.

23. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a base having a Bucky grid slidably mounted therein, a solenoid mounted on the base having a plunger connected to the grid, spring means biasing said plunger whereby the grid isreciprocated in one direction by the solenoid and in the return direction by the spring means, a reversing switch associated with the grid and in electrical connection with the solenoid to energize the solenoid when the grid reaches the end of its spring return travel, a damping unit mounted on said base constituting a fluid cylinder having a piston therein and having a restricted bypass connecting opposite ends of the cylinder, a rod connecting the piston to the grid and a valve in the piston arranged to open during reciprocation of the grid by the solenoid and arranged to close during reciprocation of the grid by the spring means to cause the fluid to flow through the restricted bypass and provide a retarded rate of grid return travel.

24. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a base having a Bucky grid slidably mounted therein, an electrical power unit mounted on the base and connected to the grid, spring means biasing said power unit whereby the grid is reciprocated in one direction by the power unit and in the return direction by the spring means, a reversing switch associated with the grid and in electrical connection with the power unit to energize the same when the grid reaches the end of its spring return travel, a damping unit mounted on said base, the damping unit constituting a fluid cylinder having a piston therein and having a bypass connecting opposite ends of the cylinder, a restrictive flow valve in the bypass to regulate the rate of piston travel, a rod connecting the piston to the grid, a valve seat in the piston and a valve element movable relative to the seat and connected to said rod, the valve element being arranged to be unseated when the rod is pulled outwardly of the cylinder during reciprocation of the grid by power unit to bypass the fluid and arranged to close when the grid is reciprocated by the spring means to cause the fluid to flow through the bypass and restrictive flow valve to provide a retarded rate of grid return travel.

25. An apparatus for reciprocating a Bucky grid during an X-ray exposure comprising a base having a Bucky grid slidably mounted therein, an electrical power unit mounted on the base and connected to the grid, spring means biasing said power unit whereby the grid is reciprocated in one direction by the power unit and in the return direction by the spring means, a reversing switch associated with the grid and in electrical connection with the power unit to energize the same when the grid reaches the end of its spring return travel, a damping unit mounted on said base, the damping unit constituting a fluid cylinder having a piston therein and having a restricted bypass connecting opposite ends of the cylinder, a rod connecting the piston to the grid and a valve in the piston arranged to open during reciprocation of the grid by the power unit and arranged to close during reciprocation of the grid by the spring means, to cause the fluid to flow through the re strict-ed bypass and provide a retarded rate of grid return travel, the internal bore of the cylinder being slotted longitudinally to provide a fluid bypass around the piston, said slots being positioned in the portion of the cylinder occupied by the piston at the beginning of the spring return travel of the grid to provide a uniform rate of grid travel during reciprocation by the spring means.

CARL K. GIERINGER. RICHARD DUCKERING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,722,573 Hirsch July 30, 1929 1,768,769 Kelley July 1, 1930 1,941,989 Larsen Jan. 2, 1934 1,945,499 Flarsheim Jan. 30, 1934 2,145,868 Fischer Feb. 7, 1939 2,147,601 Flarsheim Feb. 14, 1939 2,204,347 Files June 11, 1940 2,241,516 Ledin May 13, 1941 2,441,324 Morgan et al May 11, 1948 FOREIGN PATENTS Number Country Date 637,579 Germany Jan. 28, 1937 

